1. Field of the Invention
This invention concerns sewer back up alarms particularly usable with residential plumbing systems to give early notice of a blocked sewer condition to avoid a backup into the house.
It has long been recognized that a simple and reliable alarm would be highly desirable, and many devices have heretofore been proposed in the prior art.
However, a practical, completely acceptable device has yet to be introduced.
A sewer cap having a built in water sensor is described in U.S. Pat. No. 4,392,128. This device utilized electrical sensors which sense when water rises into an adaptor. The location of the sensor is such that the blockage must have caused the water level to rise into the adaptor to trigger an alarm, at which point it may be too late to prevent flooding. Also, the use of electrical conductors requires that they be kept clean so that the water rise may reliably set off the alarm.
U.S. Pat. No. 3,757,316 shows a similar device which optionally uses a float.
Floats have the additional disadvantage of being subject to mechanical failure and are more difficult to install into a sewer line.
U.S. Pat. Nos. 3,774,187, 4,398,186 and 4,091,365 also show float operated devices.
U.S. Pat. No. 4,546,346 describes a back up alarm which uses a built in pressure switch activated by a pressure rise generated by a back up in the sewer line.
This device depends on an air tight sealing of the threads on the cap, which is not always achieved, to compromise the reliability of the design. Again, the water level may have to rise into the branch to an appreciable height to trigger the alarm.